Propeller blades are designed to generate the maximum power at the minimum cost. The design of those blades is driven primarily by aerodynamic requirements. However, economics requires that the blade shape constitute a compromise to optimize the cost of construction versus the value of power production. The blade design process starts with a “best guess” compromise between aerodynamic and structural efficiency. The choice of materials and manufacturing process will also have an influence on how thin (hence aerodynamically ideal) the blade can be built (e.g., carbon fiber is stiffer and stronger than infused glass fiber). The chosen aerodynamic shape gives rise to loads, which are fed into the structural design. Problems identified at this stage can then be used to modify the shape, if necessary, and recalculate the aerodynamic performance.
With respect to power derived from wind turbines, the available power varies as the cube of the wind speed—accordingly, twice the wind speed equals eight times the power. Typically, wind speeds below about 5 m/s (10 mph) do not create sufficient power to be useful. Conversely, strong gusts provide extremely high levels of power. However, it is not economically viable to build turbines to optimize power peaks as their capacity would be wasted during intervals between gusts. In addition to day-to-day variations in wind power, the wind is subject to instantaneous variability due to turbulence caused by land features, thermal influences, and weather. Moreover, wind velocity tends to be greater above the ground due to surface friction. All these effects lead to varying loads on the blades of a turbine as they rotate.
The turbine itself has an effect on the wind. Downwind of the turbine, air moves more slowly than upwind. The wind starts to slow down even before it reaches the blades, reducing the wind speed through the “disc” (the imaginary circle formed by the blade tips, also called the swept area) and hence reducing the available power. Some of the wind traveling in the direction of the disc diverts around the slower-moving air and misses the blades entirely. Thus, there is an optimum amount of power to extract from a given disc diameter (i.e., if one attempts to take too much and the wind will slow down too much, reducing the available power). In fact, in a traditional wind mill configuration, it is believed than an ideal design would reduce the wind speed by about two thirds downwind of the turbine, though even then the wind just before the turbine will have lost about a third of its speed. This allows a theoretical maximum of 59% of the wind's power to be captured (referred to as Betz's limit). It is believed that in practice only 40-50% of the winds available power is captured by current designs.